This invention comprises an ultrasonic angular measurement system for the purpose of measuring the tip-off angle of a cargo pallet as it is being extracted from a cargo aircraft in flight. The tip-off angle is the angle between the deck of the aircraft and the cargo pallet. Generally, a parachute is used to pull the pallet out of the open rear cargo doors of the aircraft, and as the pallet exits, it begins to tip, and the tip-off angle increases until the pallet leaves the aircraft. It is important to measure the tip-off angle in order to determine the stability and consequent safety of various pallet loading schemes. If the pallet tip too rapidly, or too much, it can become entangled in parachute lines or damage the aircraft. Each loading scheme must, therefore, be tested and qualified before operational use. Consequently there is a need for a system which can accurately measure and record cargo pallet tip-off angles.
Various methods have been proposed to measure angular displacement using optical or ultrasonic methods. Optical angular measurement devices have been demonstrated involving reflective surfaces, optical code plate sensors, encoded diffraction grating, and photodiode arrays. The optical methods all require modification of the object to be measured (such as attaching mirrors) or contact with the object (such as connection to a shaft). Ultrasonic methods have proven more useful in applications where contact with or modification of the object to be measured is not possible.
One system used for measuring the tip-off angles during pallet drops from an aircraft uses a spring loaded swiveling arm just past the rear edge of the cargo deck. The arm is connected to a potentiometer so that as the pallet begins to tip, it moves the potentiometer to develop a voltage proportional to angle. This system has many disadvantages. First it is not as accurate as required. Further, when the pallet initially contacts the swivelling arm, it is travelling at a fairly high velocity, so that the initial shock to the arm causes it to bounce past the desired position, and the bouncing and ringing in the arm caused by the return spring cause the initial data to be inaccurate. Moreover, the mechanical assembly must be attached to the trailing edge of the aircraft, and then wires must be run to it for power, data recording, etc. These modifications to the aircraft can interfere with normal cargo door operation, and therefor usually require a specially modified test aircraft. In addition, the mechanical assembly and wiring can catch on the exiting pallet and be torn loose from the aircraft.
Another system for measuring tip-off angles uses a high speed video camera mounted inside the cargo aircraft looking at the side of the pallet, or is carried by another aircraft flying along side the cargo aircraft. In either case, the goal is to obtain a perfect side view of the pallet and cargo deck combination. The recorded video is the projected onto a "white board", one frame at a time. For each frame, a line is manually drawn on the white board corresponding to the cargo deck, and another is drawn along the edge of the cargo pallet. Then a protractor is used to measure the angle between the two lines. The angle measurement process is repeated for each frame. Because of vibration and lighting in the aircraft, the images recorded by the camera can be somewhat blurred. The movement of the aircraft and pallet can also make the optimum side view image difficult to obtain. These problem combine to make it difficult to determine where to draw the line on the white board during data reduction, and consequently, the angle accuracy is limited to 3-5 degrees. Also, the drawing of lines and measuring angles on a frame by frame basis can be long and tedious.
Because of the inadequacies of the known prior art system, there has been a need for a better system for measuring tip-off angles. The ultrasonic angular measurement system provided in accordance with this invention is capable of accuracies better than one-half degree. The disclosed system is completely self contained, battery powered so that no wiring is required, and provides automatic data recording. After the pallet drop, the data can be transferred in digital format directly to a computer which can complete all the required calculations within minutes. Furthermore, installation of the disclosed system requires no modification, or at most minimal modification to the aircraft, and the system is easily transferred to other aircraft.